Polymers, like polypropylene, are increasingly used in different demanding applications. At the same time there is a continuous search for tailored polymers which meet the requirements of these applications. The demands can be challenging, since many polymer properties are directly or indirectly interrelated, i.e. improving a specific property can only be accomplished on the expense of another property. Stiffness can for instance be improved by increasing the crystallinity and/or the relative amount of homopolymer within the composition. As a consequence, the material becomes more brittle, thereby resulting in poor impact properties. It is known that impact strength of polypropylene can be improved by dispersing a rubber phase within the polymer matrix, thereby obtaining a heterophasic polypropylene composition.
Such heterophasic propylene copolymers comprise a matrix being either a propylene homopolymer or a random propylene copolymer in which an amorphous phase, which contains a propylene copolymer rubber (elastomer), is dispersed. Thus the polypropylene matrix contains (finely) dispersed inclusions not being part of the matrix and said inclusions contain the elastomer. The term inclusion indicates that the matrix and the inclusion form different phases within the heterophasic propylene copolymer, said inclusions are for instance visible by high resolution microscopy, like electron microscopy or scanning force microscopy or atomic force microscopy, or by dynamic mechanical thermal analysis (DMTA). Further the heterophasic polypropylene may contain to some extent a crystalline polyethylene, which is a by-reaction product obtained by the preparation of the heterophasic propylene copolymer. Such crystalline polyethylene is present as inclusion of the amorphous phase due to thermodynamic reasons.
One preferred application of such heterophasic polypropylene compositions is its use in the field of automotive applications, due to the trend to replace metal parts with plastics, i.e it is particularly desired to replace automotive parts which may be subjected to extremely low temperatures such as −30 or −40° C. still being made from metal alloys by lighter plastics. Furthermore high flow heterophasic materials are specifically desired in the automotive field.
Powder flowability, i.e. non-stickiness of the as-polymerized powder, is especially important for reactor made heterophasic high impact polypropylene compositions (RTPOs), where a polypropylene homopolymer (homo PP) matrix and the rubber as dispersed phase are produced in a multistage process with several reactors in series.
During the reaction the rubber stays in pores and cavities of the particles, which are formed in the first stage of the multistage polymerization. When the pores and cavities in the homo PP matrix are completely filled, the extra rubber starts to migrate to the surface, causing stickiness both between particles, leading to agglomeration of particles, and between particles and the reactor wall. This causes severe problems in the process.
Further nowadays the polymer processors desire material with low emissions to fulfil the consistently rising demands of regulatory authorities as well as consumers.
One further disadvantage of polypropylene resins is that in most moulding processes they are subject to significant post-moulding shrinkage. This means that in applications where dimensional tolerances are important the mould must be tailored to the specific composition and the specific moulding operation to yield a finished part of the precise dimension which is required. This shrinkage problem is particularly troublesome where the manufacturer has moulds tailored to a certain composition and moulding operation and subsequently wishes to substitute a different composition or halter the process to e.g. increase the cooling rate. This problem is enhanced because polypropylene resins show a post-moulding shrinkage difference in the longitudinal and the transversal direction.
Although a lot of development work has been done in the field of heterophasic polypropylene compositions, it was up to now not possible to find a well-balanced polymer composition with respect to impact strength and stiffness in combination with high powder flowability, low emissions and low shrinkage.
Therefore, there is still a need for a heterophasic polypropylene composition which while keeping acceptable (low temperature) impact resistance has high stiffness and high powder flowability in combination with low emissions and low shrinkage.
Hence, it is an object of the present invention to provide such a material.